The present invention relates to a process for the detoxification of effluent obtained from a variety of industrial processes, using hydrogen peroxide and trimercaptotriazine.
Purification of waste waters and recovery of components thereof is of major importance in connection with protection of the environment. Effluents from a variety of industries, including electroplating, steel-hardening and mining operations, often contain cyanide in various forms together with toxic metals. The detoxification of cyanide and the recovery of the toxic metals, or their conversion into harmless forms, are of paramount importance before such waste waters are discharged, in order to avoid serious damage to the environment and adverse impact on public health and safety.
A number of methods have been developed over the years for the treatment of such effluents. One of the best of the known methods involves the oxidation of the cyanide contained in the waste water with hydrogen peroxide. As far as possible, heavy metals contained in such waste effluents are removed by precipitation as a part of the overall process. These methods have been used successfully on a commercial scale for a number of years and are known to have a number of advantages over the even older methods such as treatment with chlorine, because the excess hydrogen peroxide that is utilized decomposes to give only water and oxygen. In the older methods involving chlorine, undesirable salts are formed and introduced into the waste water. The utilization of hydrogen peroxide overcomes the disadvantages associated with methods which involve salt formation and the introduction of those salts into waste waters.
Of all the forms in which cyanide occurs in waste waters, hydrogen peroxide is capable of oxidizing free cyanide ions and certain of the complex metal cyanides, including the tetracyanozincate, tetracyanocadmate, dicyanocuprate (I), tricyanocuprate (I) and tetracyanocuprate (I) ions. Species which are not oxidized to a significant or sufficient extent by hydrogen peroxide alone include hexacyanoferrate (II), hexacyanoferrate (III) and hexacyanocobaltate (III). One species that is often difficult to oxidize is the tetracyanonickelate (II) ion.
It can therefore happen that treatment of a waste water with hydrogen peroxide alone does not result in complete detoxification, due to the presence of cyanide in a form which is not amenable to oxidation.
In the case of hexacyanoferrate (II) ions, it is possible to remove these by precipitation with copper (II) ions, and separate the resulting copper (II) hexacyanoferrate (II) from the waste water by settling or filtration. Nevertheless, waste waters are sometimes encountered which are very difficult to treat, owing to the presence of complex metal cyanides which are not amenable to oxidation.
One method of removing toxic metals from waste waters is to add a chemical substance that induces the formation of an insoluble metal compound. This compound then forms a precipitate which can be separated from the waste water by settling or filtration. After separation, the clean waste water can be discharged and the precipitate can be recovered for re-use or safe disposal. The simplest and most frequently-used chemical substance used to precipitate toxic metals from waste water is an alkali such as lime, which precipitates metals in the form of their hydroxides. However, this method fails when complexing agents are present in the waste water, which prevent the precipitation. Various agents have been proposed or are in use for the purpose of precipitating metals from waste water containing complexing agents. These include solutions of sodium sulfide or sodium hydrosulfide, salts and esters of dithiocarbamic acid, and trimercaptotriazine (TMT) in its free acid or trisodium forms. These agents are capable of precipitating some metals from waste waters containing weak complexing agents such as ammonia or chloride, but they are less effective at precipitating metals from solutions containing cyanide, owing to the great complexing power of the cyanide ion. Solutions containing both cyanide and heavy metals are usually treated by oxidation of the cyanide, as described above, but if metal cyanide complexes are present which cannot be oxidized, the metals stay in solution and resist precipitation.
One method of treating waste waters containing cyanide and metals has been described in U.S. Pat. No. 4,417,987, whereby hydrogen peroxide is used in the presence of a complexing agent such as ethylenediamino-tetra acetic acid. This method suffers from the considerable disadvantage that the toxic metal is not removed from the waste water, but remains held in solution by the complexing agent.
Trimercaptotriazine is one precipitating agent which is commercially available in the free acid form or as the trisodium salt. Its structure may be represented as follows: ##STR1##
Trimercaptotriazine has a very low toxicity to fish, and the compounds it forms with heavy metals such as copper, nickel or mercury, are highly insoluble and highly stable. These properties make trimercaptotriazine more ecologically acceptable than other precipitating agents, such as sodium sulfide.